This invention relates to the mounting of knob or lever assemblies on doors and similar members which are retained in a particular position by a latch, deadbolt, or the like. It will be hereinafter convenient to describe the invention with particular reference to doors and latches, although as indicated above the invention has wider application. Thus, the word "door" as hereinafter used throughout this specification is to be understood as embracing other forms of support members to which a knob or lever assembly of the foregoing kind may be mounted.
Door sets of the foregoing kind generally include a knob or lever which is rotatably mounted on a rose or other mounting plate or assembly adapted to be secured to a surface of the door. A drive spindle connects that knob or lever with another knob or lever at the opposite side of the door. The knob or lever at one side of the door at least may include key operated locking mechanism which must be released in order for the knob or lever to be operable to actuate an associated latch assembly. In many cases the latch assembly includes a tubular housing which is mounted within a bore formed in an edge of the door, and a latch bolt slidably mounted in that housing for movement between projecting and retracted positions at which it can and cannot respectively cooperate with a strike secured to the door frame. The aforementioned drive spindle passes through the latch housing and cooperates with mechanism carried by that housing so that the latch bolt retracts in response to rotation of one of the knobs or levers.
It is generally the case that the diameter of the bore required to receive the latch housing is substantially less than that required to mount the associated door sets--i.e., the knob or lever assemblies. For example, the bore within the door edge for receiving the latch housing may be in the order of five eighths of an inch diameter, whereas a two and one eighth inch diameter hole extending transversely through the door and intersecting with the aforementioned bore may be required to mount the door sets. Special tools have been developed for producing such bores and holes of the diameters mentioned and because of the convenience of using those standard tools tradesmen commonly use the two and one eighth inch diameter boring tool even in situations where a smaller diameter hole may be adequate.
The aforementioned practice presents problems in situations where the door is required to have a standard suitable for fire installation purposes. A door set mounted in such a large diameter hole will usually be unacceptable for such purposes because if one of the door sets melts under fire conditions the other will be unsupported and can therefore fall away leaving a relatively large opening through which fire and smoke can pass.
One method of meeting the foregoing problem has involved providing a relatively small diameter hole through the door to accept the drive spindle and then counterboring from each side of the door to provide a large diameter cavity to mount the door sets. That has the disadvantage of requiring several boring operations and it is found in practice that some tradesmen tend to ignore the safety requirements and bore through the door in a single pass using the large diameter boring tool.
Yet another problem arises in hollow doors having fire insulating material between their outer face panels. Such material is commonly of a loose nature and it tends to fall into the space created by the through hole and latch housing bore and from there it can enter the latch and door lock mechanism and thereby disturb the operation of that mechanism.